Contaminants represent undesirable components in a pulp suspension or in white waters because they may hamper manufacturing operations or quality of pulp, paper or paper products. Depending on a pre-established size scale, they are classified as macrocontaminants and microcontaminants. It is customary to distinguish macro- from microcontaminants by measuring the amount of material retained on a 0.10 to 0.15 mm slotted screen. Accurate determination of macrocontaminants in pulps and white waters is of utmost importance for pulp and papermakers, chemical suppliers, adhesives manufacturers, printers and researchers.
This measurement of macrocontaminants is particularly useful for a pulp and paper mill because it dictates control and elimination strategies. The nature and amount of contaminants in pulps and water streams depend on the type of pulp manufactured and on the degree of system closure. An example of harmful contaminants in virgin pulp is shines originating from incomplete defiberizing of wood. Stickies are another important class of contaminants so-named because they pertain to materials that have a strong tendency to deposit on or stick to a wide variety of surfaces.
Stickies, which represent by far the most detrimental class of contaminants in recycled pulps, embrace a large variety of lipophilic compounds. They originate from pressure sensitive adhesives (PSAs), hot melts, toner, waxes, coating and binders that are used in labels, tapes, envelopes, stamps, paperboards for a variety of functions like binding, sealing, coating and printing. Adhesives, coatings and waxes, which are introduced in recycling plants with recovered papers, are subjected to intense mechanical action during pulping, screening, pumping, dispersion and kneading. Strong shear forces applied to adhesives during pulp processing and paper manufacturing will lead to their fragmentation into a wide range of particle sizes. The release of sticky substances in the water phase leads to the production of off spec pulp from the drinking plant, increased chemical costs for their passivation and solvent cleaning, poor runability at the paper machine and in press rooms, and poor product quality. Though vital for efficient control of recycling operations macrostickies measurement is infrequent, due to personnel shortages and the tedious nature of the manual methods [Refs: 1-5]. Despite the seriousness of the problem, very few devices have been proposed for on-line monitoring stickies in pulp streams [Refs: 6-10].
One of the biggest hurdles faced in developing an on-line instrument is that the unit must be capable of measuring a small number of contaminants in a pulp mat or in a pulp suspension consisting mainly of fibres. The presence of these fibres impedes proper detection of contaminants, especially when image analysis is used for objects discrimination and quantification. To get around this problem, a wide variety of laboratory methods first separate contaminants prior to their detection, quantification and measurement. The most widely-used laboratory methods use laboratory screens, hydrocyclones, filtration and the like to first isolate the contaminants from the fibre [Refs: 1-5, 10]. Once segregated, the contaminants, such as stickies, dirt, and shines are collected from the screen or deposited on filter paper. Whereas dirt and shives do not require special preparation for their quantification, it is a different matter when the contaminants include stickies. The tacky nature of stickies is used to separate them from other macrocontaminants or to transfer them to an appropriate support where they can be examined visually and counted by eye or with the aid of an image analyser. The preparation of stickies for visual analysis is a tedious procedure that limits the throughput of stickies analysis. Some other instruments are based on the tackiness of the contaminants and their tendency to deposit on paper machine wires and paper [Refs: 2, 11-14]. Although useful, these laboratory tests are all time consuming and without microscopy and/or chemical analysis do not reveal much about the type of contaminant, their area or numbers.
A few patents exist that describe equipment for on-line measurement of small proportion of components in a liquid containing predominantly other components, particularly in the area of detecting white blood cells, platelets or antibodies in blood [Refs; 15-31] and shives in pulp [Refs: 32-43]. Fewer patents refer to the actual on-line measurement of stickies in pulp [Refs: 6-10, 44].
Many of these analysers require dilution of blood or pulp samples to facilitate visualization or detection of the component of interest. Passage through laminar hydrodynamic flow cells then allow component or contaminant identification, measurement or enumeration [Refs: 15, 17-19, 21-32, 34-36, 41-69]. Visualization of white blood cells or platelets in a mass of red blood cells often involve sphering the red blood cells or promoting their removal through cell lysis [Refs: 15, 18, 22, 23, 28, 30, 50, 51, 59], staining of different biological components with fluorescent dyes [Refs: 18-22, 24, 25, 27, 30, 70, 71], or their separation through electrophoresis [Refs: 72-77]. In these systems, particle detection can be made through photodetection of scattered light [Refs: 15, 18, 21-25, 27, 30, 31, 70, 71, 78, 79], near infrared reflectance [Refs: 56, 57] or by image analysis of captured images by charged coupled devices [Refs: 25-27, 49, 52-55, 72, 76, 77, 80-82]. Image analysis is a useful tool that discerns features of individual components and allows their enumeration and measurement [Refs: 25, 80]. Unlike blood cells, pulp fibres, shives or stickies cannot undergo cell lysis to separate white blood cells and platelets from the larger mass of red blood cells. On the other hand, in many respects many of the techniques used to separate, detect and measure white blood cells and platelets can be used for pulp. Sample dilution, hydrodynamic focusing and the use of fluorescent dyes are indeed used in many pulp applications to separate and/or distinguish fibres prior to their photodetection or imaging by charged coupled devices. [Refs: 6, 7, 9, 32, 34, 35, 42-44, 83]. However, when the object of analysis is a large sticky particle, its amount relative to fibres population is so small that a large volume of sample must be processed at a low flow rate in the laminar flow cell in order to detect sufficient amount of stickies to obtain statistically significant counts. This imposes serious limitations to the throughput of an analyser using hydrodynamic focusing. These methods are further described in the following paragraphs.
Compared to blood analysers, shine analysers use pulp dilution, hydrodynamic focusing, screens, hydrocyclones or suction extractor through gaps to separate the shives from the fibre mass prior to analysis of contaminants [Refs: 9, 32, 34-43, 84]. Because shives have wider diameters and higher densities than water, screens and hydrocyclones can isolate shives from the pulp mass. Light is passed through the flow cell and the sample and, the change in light or pulse signal will allow determination of particle or fibre size [Refs: 9, 32, 34-36, 40, 42, 43]. Although useful, the small number of contaminants and their similar color to the fibre hampers their visualization and identification. The same principles that allow shives content determination—higher density than water and greater diameter than fibres—will also facilitate segregation of macrostickies from pulp.
For stickies detection and measurement, four methods using fluorescent dyes to detect hydrophobic components of pulp have been described [Refs: 6-8, 83]. The methods of Horn et al. [7] was designed to detect wood resin particles in pulp. This analyser used the same principles as those analysers described by Esser et al. [8], Di Cesare [6] and Perry et al. [83] except that they are defined as stickies analysers. Each of these instruments use photodetection coupled with the addition of fluorescent dyes to a pulp suspension as a means to identify organic components such as stickies and wood resin in pulp. The fluorescent dye reacts with the hydrophobic contaminants such as stickies. When the fluorescent-dyed components are excited by light at a specified wavelength, the hydrophobic contaminants will emit light which can later be detected by a photodetector. The light emission signals proportional to the size of the contaminant is detected in the photocell and the signals evaluated to measure the hydrophobic contaminants. One of the problems with this method, is that fibres, fines and shives rich in lipophilic extractives such as triglycerides and fatty acids [85] may interfere in the measurement of stickies. Hence, these methods do not distinguish between stickies and pitch because both classes are hydrophobic in nature. Moreover, these methods do not permit enumeration of stickies based on their area or type. All of these four methods use laminar flow cells or hydrodynamic focusing to separate the contaminants and allow their detection. Combining hydrodynamic focusing with a prior contaminant isolation step might give better results such as the method described by Carr [9].
The first stickies analyser was described by Carr [9]. This analyser functions by first diluting the pulp sample to a consistency less than 0.5% and passing the diluted pulp sample through a series of hydrocyclones to separate particles according to their density. The separated particles are then diluted again to a consistency less than 0.5% before passage through a flow cell. The flow is back lighted and a photodetector includes a linear array of sensitive elements aligned to receive the transmitted light. The sensitive elements aligned with particles create a signal proportional to the width. By rapid sequential activation of the elements, a digital data stream is created which is processed by a microprocessor to determine the particle size and produce a plurality of contaminant relative signals related to different classified size ranges, such as heavy, medium and small contaminant particles. The addition of a fluorescent dye could also be added to help identify contaminants in the photocell. If we take a new look at this method over 20 years later, the use of charged coupled devices with added imaging capacities may improve this method. One of the limits of the method is that hydrocyclones are used to separate stickies from pulp. With the exception of waxes and other light-weight macrocontaminants, often macrostickies have a similar density to water, limiting their isolation via hydrocyclones and leading to the presence of fibres in the sample preparation. Another drawback of the method as an on-line device is that it uses hydrodynamic focusing to separate particles in a laminar flow prior to their detection. The sample size that can be handled by such cells is small in the order of milliliters. Passage of liter-sized samples resulting from pulp screening or cleaning methods would limit use of such flow cells.
A method for analyzing very small stickies in pulps has been described by Banerjee [10]. Again screening, albeit through filtration, is used to remove fibres, fibre debris, and other large contaminant particles from the fibre slurry, after which the carbon content of the filtered sample is measured. Next, the filtrate is ultrafiltered to separate stickies having a high molecular weight from the filtrate, and subsequently, the carbon content of the ultrafiltered sample is measured. The filter pore size used in ultrafiltration is of 25 μm. Finally, the carbon contents are used to determine the microstickies concentration in the fibre slurry. Although useful, this method is reported to give an estimate of microstickies content and not that of macrocontaminants such as macrostickies, shines and toner. In principle, other organic material solubilized from wood or arising from paper machine additives, should pass through the pores of 25 μm and not be detected. However, these components tend to form agglomerates in pulp waters that may interfere with this measurement.
Flow cells have been known to measure dark particles or specks such as bark, metal, and toner contaminants present in kraft [84] and recycled pulps (Simpatic, PapTech). Although good for measuring dark contaminants [38, 84], these analysers have trouble detecting contaminants that are not visually very different from the pulp, such as stickies [86].